1. Field of the Invention
The present invention relates to a liquid crystal display device, and more specifically to a technique suitable for a liquid crystal display device formed by sticking substrates together in accordance with a one drop fill method.
2. Description of the Related Art
A liquid crystal display device has a structure in which liquid crystal is interposed between two glass substrates disposed to face each other and stuck together by using an adhesive. To achieve a uniform cell gap (a thickness of a liquid crystal layer), plastic beads having even grain sizes are dispersed between the substrates. In recent years, there is known a technique to maintain a uniform cell gap by forming protrusions (spacers) on a substrate. The spacers can be selectively arranged and also have high accuracy in height thereof. Recently, there has been disclosed a technique for eliminating unevenness between substrates caused by disposing spacers having the same height in a central part and a peripheral part, which have different lamination structures, of the substrates (see Japanese Patent No. 3388463, for example).
Meanwhile, techniques for sticking the substrates together include a vacuum injection method and a one drop fill method. According to the vacuum injection method, an adhesive is disposed on one of the substrates while forming a liquid crystal inlet therein. After the two substrates are assembled together, a cell gap is formed by crushing the adhesive while evacuating air between the substrates from the liquid crystal inlet with a load applied onto a substrate surface, or with a vacuum state established.
On the other hand, according to the one drip fill method, a given amount of liquid crystal is dropped into a region surrounded by an adhesive disposed on one of the substrates and the two substrates are assembled together in a vacuum state. Thereafter, an atmospheric pressure is recovered so that the adhesive is crushed by a difference in pressure between the inside and the outside of the region surrounded by the two substrates and an inner wall of the adhesive. As a result, a predetermined cell gap is formed between the substrates.
In recent years, the one drop fill method is widely diffused because of its capabilities of shortening tact time, improving use efficiency of materials, achieving higher accuracy in the cell gap, and so forth (see Japanese Unexamined Patent Application Publication No. Hei 5-232481, for example).
However, in the conventional liquid crystal display device, degrees of irregularities are significantly different between a display region where electrode wiring, coloring layers of color filters, and the like are densely formed and a periphery (a peripheral region) of the display region. For this reason, if the spacers formed on the substrate have uniform heights, the spacers in the display region contact the opposed substrate but the spacers in the peripheral region do not contact the opposed substrate. Accordingly, there are a portion where the spacers function and a portion where the spacers do not function.
If the substrates are assembled together in this state, the cell gap in the peripheral region becomes thinner than the cell gap in the display region. Moreover, at a part of the display region located close to the peripheral region, one of the substrates is deformed and the cell gap becomes locally thicker. As a result, the cell gap at the part of display region located close to the peripheral region becomes different from the cell gap at a central part of the display region and such a difference is recognized as unevenness of display. Particularly, in the one drop fill method, the pressure is applied evenly to the substrate surface when the substrates are assembled together. Accordingly, this problem becomes more apparent when the spacers do not uniformly support the space between the substrates because the crushing manner of the adhesive is apt to be uneven.
Incidentally, the cell gap is influenced by an amount of injection of the liquid crystal. In the vacuum injection method configured to inject the liquid crystal from the inlet, the liquid crystal is hardly injected in excess of a volume inside a liquid crystal cell. Even if the liquid crystal is excessively injected, it is possible to squeeze the excessive portion out by adjusting the pressure. Accordingly, the vacuum injection method hardly causes an abnormal cell gap. On the contrary, in the one drop fill method, the liquid crystal is dropped on the closed region surrounded by the adhesive without the inlet and then the substrates are assembled together. Therefore, the cell gap becomes larger if the dropped amount of the liquid crystal is excessive. On the other hand, if the dropped amount of the liquid crystal is inadequate, the cell gap becomes smaller due to a shortage of the liquid crystal and bubbles may be generated. As described above, the one drop fill method has a problem of causing uneven cell gaps attributable to the unevenness in the amount of injection of the liquid crystal cell.